Generally, secondary batteries are batteries that can be charged and discharged. In contrast, primary batteries are those that cannot be charged. Secondary batteries are widely used in advanced electronic devices, including cellular phones, notebook computers, camcorders, etc. Particularly, lithium secondary batteries have driving voltages of 3.6 V or more. These driving voltages are three times greater than those of nickel-cadmium (Ni—Cd) batteries or nickel-metal hydride (Ni-MH) batteries, which are currently used as power sources for portable electronic devices. Further, lithium secondary batteries have relatively high energy densities per unit mass. Therefore, lithium ion secondary batteries have been increasingly and extensively researched and developed.
Lithium secondary batteries have conventionally used lithium-containing oxides as cathode active materials and carbonaceous materials as anode active materials. Generally, lithium secondary batteries are classified into liquid electrolyte-based batteries and polymer electrolyte-based batteries, according to the type of electrolyte used. Batteries using a liquid electrolyte are referred to as lithium ion batteries and batteries using a polymer electrolyte are referred to as lithium polymer batteries.
A lithium secondary battery generally comprises an electrode assembly comprising a cathode, an anode and a separator positioned between the cathode and anode. The lithium secondary battery is formed by winding the electrode assembly into a jelly roll shape. The wound electrode assembly is then inserted into a can which generally comprises aluminum or an aluminum alloy. The can is then closed with a cap assembly, and an electrolyte is injected into the can. The can is then sealed. The battery further comprises a sheath on the outer surface of the can. The sheath protects and insulates the battery from the external environment. Generally, this insulating sheath comprises a material that shrinks with heat, such as polyvinyl chloride (PVC). The sheath is attached to the battery by heating, thereby creating a complete battery.
Batteries are energy sources and have the potential to discharge large amounts of energy. Secondary batteries store large amounts of energy during charging. Also, secondary batteries require external energy sources during charging for supplying the energy to be stored in the battery. When abnormalities occur during charging a secondary battery, such as internal short circuits, the battery may rapidly discharge the stored energy, causing safety-related problems such as fire, explosion, and the like.
Additionally, due to the high activity of Li, lithium secondary batteries may undergo rapid exothermic reactions when exposed to small amounts of water contained in an electrolyte, as depicted in Formula (1) below. When it is not exposed to water, lithium reduces and precipitates during charging and discharging, as depicted in Formula (2) below. This reduction and precipitation of lithium causes short circuits between the cathode and anode. Therefore, lithium secondary batteries which do not charge or discharge property create high risks of fire or explosion.Li+H2O→LiOH+½H2↑  (1)LiLi++e−(forward reaction:dissolution, reverse reaction:precipitation)  (2)
Accordingly, secondary batteries are generally equipped with various safety devices for preventing fire or explosion caused by abnormal charging. For example, certain safety devices interrupt electric current when the battery temperature is too high or when the voltage increases too rapidly due to overcharging or over-discharging. Such safety devices prevent dangers such as explosion and fire. Some examples of safety devices include protective circuit boards that detect abnormal electric current or voltage and that interrupt electric current, Positive Temperature Coefficient (PTC) devices that are activated upon overheating due to abnormal electric current, and bimetal devices.
However, even when secondary batteries are equipped with such safety devices, users cannot visually detect abnormal and rapid increases in temperature during charging and discharging. Therefore, users cannot easily determine whether the battery is operating improperly. Additionally, secondary batteries can be misused in several ways, including user misuse, use with a malfunctioning charger, and use contrary to prescriptions for normal use. When such misuse occurs, for example during overcharging, the battery may ignite or explode. Further, contacting a coin or necklace with an external connection terminal may cause the battery to ignite or explode. However, it is difficult to detect these abnormalities so as to prevent dangerous occurrences such as battery breakdown and ignition.